This invention relates to an apparatus for compressing a picture signal, and an apparatus for expanding a compressed picture signal, and, more particularly, to an apparatus that is suitable for use in applications in which the picture signal being compressed is to be recorded, and in which the compressed picture signal being expanded has been reproduced.
It is known to compress a picture signal by dividing the picture signal into blocks of 8.times.8 pixels (=8 pixels.times.8 lines), and subjecting the blocks to processing by means of a discrete cosine transform (DCT), quantizing, and variable length coding the resulting transform coefficients, which are then recorded on a recording medium, e.g. a disc. The compressed picture signal recorded on the disc is then reproduced from the disc, variable length decoded, inverse-quantized, and inverse-discrete cosine transformed to reconstruct the original picture signal.
It is desirable to provide a recording medium that has a short access time and a large capacity because a motion picture, for example, requires that a large quantity of information to be stored. Presently, an NTSC video signal, for example, can be recorded on and reproduced from a conventional video disc. When it is desired to record the digital motion picture signal on a smaller disc than a conventional video disc, the motion picture signal must be subject to high efficiency compression, and the reproduced motion pictures signal must be capable of being expanded efficiently.
To answer this problem, there have been proposed some methods for compressing the motion picture signal to be recorded with high efficiency. One of these methods is that proposed by the moving picture experts group (MPEG). The MPEG method detects a motion vector for each block of the motion picture signal, and generates a prediction block by applying motion compensation to a prediction picture according to the motion vector. This reduces redundancy in the motion picture signal in the time domain. In addition, the block of prediction errors between each block of the present picture and its corresponding prediction block is subject to a discrete cosine transform, and the resulting transform coefficients are quantized, to reduce redundancy in the motion picture signal in the spatial domain.
Attempting to increase the compression efficiency by enlarging the quantizing step size by which the transform coefficients are quantized results in larger quantizing errors. Larger quantizing errors make noise the in flat portions of the picture (i.e. the portions of the picture in which there is little detail) more obvious.
Further, in a conventional apparatus for compressing a motion picture signal, the differential vector between the motion vectors of the target block and the left side block thereof is encoded on encoding the motion vector of prescribed each block. Therefore, when there are many targets to be imaged in a picture, the motions of which are different each other, the quantity of prediction error information between the current picture and the prediction picture is increased, which degrades the compression efficiency.
Furthermore, in this situation, there is a problem that different parts of the block can have motions that are different from each other, which degrades the prediction accuracy.
To remedy this problem, it has been suggested that each of the 8.times.8 blocks be divided into four 4.times.4 subblocks, that a motion vector be determined for each subblock, and that the motion of the block be compensated by using the resulting four motion vectors. However this proposal degrades the compression efficiency because of the increased number of motion vectors.